1. Field of the Invention
The present invention relates generally to joint structures between top nozzles and guide thimbles of nuclear fuel assemblies and, more particularly, to a joint structure between a top nozzle and a guide thimble which is configured such that an inner-extension tube is prevented from undesirably rotating when the top nozzle is separated from the nuclear fuel assembly, and assembly and disassembly between the top nozzle and the guide thimble is facilitated.
2. Description of the Related Art
A nuclear reactor refers to a device that is designed to exert artificial control over the chain reaction of the nuclear fission of fissile materials, thereby achieving a variety of purposes such as the generation of heat, the production of radioisotopes and plutonium, the formation of radiation fields, or the like.
Generally, enriched uranium that is obtained by raising the ratio of uranium-235 to a range between 2% and 5% is used in a light water nuclear reactor. The uranium is molded into a cylindrical pellet that weighs 5 g and processed into nuclear fuel that is used in a nuclear reactor. Numerous pellets are embedded into a cladding tube made of Zircaloy which is in a vacuum state. Thereafter, a spring and helium gas are put into the tube, and then a top end closure stopper is welded thereon, thereby making a fuel rod. A plurality of fuel rods constitutes a nuclear fuel assembly and is burned in a nuclear reactor by nuclear reaction.
FIG. 1 is a schematic view showing a general nuclear fuel assembly.
Referring to FIG. 1, the nuclear fuel assembly includes a skeleton and a plurality of fuel rods 1. The skeleton includes a top nozzle 4, a bottom nozzle 5, a plurality of spacer grids 2, a plurality of guide thimbles 3 and a instrument tube 6. The fuel rods 1 are inserted longitudinally into an organized array by the spacer grids 2 in such a manner as to be supported by means of springs (not shown) and dimples (not shown) which are formed in the spacer grids 2. In order to prevent the formation of scratches on the fuel rods 1 and damage to the springs upon assembling the nuclear fuel assembly, lacquer is applied to the surfaces of the fuel rods 1 before the fuel rods 1 are inserted longitudinally into the skeleton of the nuclear fuel assembly. Subsequently, the top and bottom nozzles are secured to the opposite ends of the nuclear fuel assembly, thereby finishing the procedure of assembly of the nuclear fuel assembly. After the lacquer is removed, the following items of the assembled nuclear fuel assembly are tested: the distance between the fuel rods, distortion, dimensions including the length, etc., thus completing the process of manufacturing the nuclear fuel assembly.
As shown in FIG. 2, the top nozzle 4 includes a hold-down plate 8, hold-down springs 43, inner-extension tubes 45, outer guide posts 44, and a flow plate 7.
Referring to FIGS. 1 and 2, the inner-extension tubes 45 of the top nozzle 4 are connected to the respective guide thimbles 3 so that the nuclear fuel assembly can be firmly fixed in the reactor and the structural stability of the nuclear fuel can be ensured during the burn-up of the nuclear fuel.
The top nozzle 4 and the guide thimbles 3 are joined to each other in such a way as to be removably connected to each other, thereby ensuring a path along which the fuel rods 1 can be drawn out when disassembling the top nozzle 4. Disassembly of the top nozzle 4 from the guide thimbles 3 is carried out in a storage tank. A worker must remotely perform the disassembly work to minimize the harm caused by radiation. Accordingly, the joint structure between the top nozzle 4 and the guide thimbles 3 must be designed such that assembly or disassembly between them can be conducted remotely.
FIGS. 2 and 3 illustrate a typical method of joining the guide thimbles 3 with the top nozzle 4. Referring to the drawings, the method of joining the guide thimbles 3 and the top nozzle 4 will be described. As shown in FIG. 2, an external thread is formed on a lower end 451 of each inner-extension tube 45. As shown in FIG. 3, an internal thread is formed on an inner surface of a threaded portion 31 of each guide thimble 3. The top nozzle 4 and the guide thimbles 3 are joined with each other by thread-coupling.
An external thread is formed on a lower end of each outer guide post 44. The outer guide posts 44 are threadedly coupled to the flow plate 7. The threaded lower end of each outer guide post 44 is partially welded to the flow plate 7 to prevent the outer guide post 44 from rotating. Furthermore, in order to prevent each inner-extension tube 45 from becoming loose, a head of the inner-extension tube 45 is partially crimped in a radial direction in such a way as to be put in contact with the outer guide post 44. Moreover, the inner-extension tube 45 can be separated from the outer guide post 44 only when torque of more than a specific strength is applied to the head.
However, in the state where the inner-extension tube 45 is joined with the outer guide post 44, when the inner-extension tube 45 of the top nozzle 4 is rotated to be separated from the outer guide post 44, since the distance between an outer surface of the inner-extension tube 45 and an inner surface of the outer guide post 44 is too short, it is difficult to rotate the inner-extension tube 45 along the threads if concentricity is not congruous or if foreign substances have gotten stuck between the outer face and the inner face. That is, due to frictional heat generated by the contact surface, the inner-extension tubes 45 and the outer guide posts 44, which are made of stainless steel, are fused together by a cold welding effect, and hence, loosening does not occur.
To solve the above-mentioned problems, there have been disclosed U.S. Pat. No. 4,702,883 entitled “Reconstitutable fuel assembly having removable upper stops on guide thimbles”, and U.S. Pat. No. 4,687,630 entitled “Top nozzle and guide thimble joint structure in a nuclear fuel assembly”.
In the prior arts, heads of outer guide posts are removed without any inner-extension tube, and processed to have threads so as to minimize the contact surface when the outer guide posts are removed. That is, thread-coupling portions are not only formed on an external threaded portion of a lower portion of each outer guide post and on an external threaded portion of a lower portion of each guide thimble, but also an additional thread-coupling portion is provided on an upper portion of the outer guide post.
Accordingly, when the head of each outer guide post is rotated to remove the top nozzle, since the outer guide post and the head thereof are threadedly-coupled with each other, the thread-coupling between the outer guide post and the guide thimble may become loosened. Hence, in order to prevent the lower end of the outer guide post from becoming loosened, the outer guide post is equipped with a wedge device; however, this has the problem of the assembling and disassembling processes being complicated.
Another conventional technique was proposed in Korean Patent Application No. 10-2007-0086066, which was filed by the applicant of the present invention and entitled “Joint structure between top nozzle and guide thimble for nuclear fuel assembly”. This technique is provided with a structure for preventing an inner-extension tube from becoming loose when the top nozzle is disassembled. However, a threaded portion is disposed on an end of each of an inner-extension tube body and the inner-extension tube head, thus making it difficult to assemble them remotely. In other words, because the threaded portions must be coupled to each other without using an introduction portion that can serve as a guide, if two elements are not coaxially aligned with each other, the thread-coupling between the two elements may fail.